Ch07 Ceramics Wiley

2002 John Wiley & Sons, Inc. M. P. Groover,Fundamentals of Modern Manufacturing 2/e

CERAMICS

Structure and Properties of CeramicsTraditional CeramicsNew CeramicsGlassSome Important Elements Related to CeramicsGuide to Processing Ceramics


Ceramic Defined

An inorganic compound consisting of a metal (orsemi-metal) and one or more nonmetals Important examples:

Silica - silicon dioxide (SiO2), the main ingredientin most glass productsAlumina - aluminum oxide (Al2O3), used in various

applications from abrasives to artificial bonesMore complex compounds such as hydrous

aluminum silicate (Al2Si2O5(OH)4), the mainingredient in most clay products


Properties of Ceramic Materials

High hardness, electrical and thermal insulating,chemical stability, and high melting temperaturesBrittle, virtually no ductility - can cause problems in

both processing and performance of ceramicproductsSome ceramics are translucent, window glass (based

on silica) being the clearest example


Ceramic Products

Clay construction products - bricks, clay pipe, andbuilding tileRefractory ceramics - ceramics capable of high

temperature applications such as furnace walls,crucibles, and moldsCement used in concrete - used for construction and

roadsWhiteware products - pottery, stoneware, fine china,

porcelain, and other tableware, based on mixtures ofclay and other minerals


Ceramic Products (continued)

Glass - bottles, glasses, lenses, window pane, andlight bulbsGlass fibers - thermal insulating wool, reinforced

plastics (fiberglass), and fiber optics communicationslinesAbrasives - aluminum oxide and silicon carbideCutting tool materials - tungsten carbide, aluminum

oxide, and cubic boron nitride


Ceramic Products (continued)

Ceramic insulators - applications include electricaltransmission components, spark plugs, andmicroelectronic chip substratesMagnetic ceramicsexample: computer memoriesNuclear fuels based on uranium oxide (UO2)Bioceramics - artificial teeth and bones


Three Basic Categories of Ceramics

1. Traditional ceramics - clay products such as potteryand bricks, common abrasives, and cement

2. New ceramics - more recently developed ceramicsbased on oxides, carbides, etc., and generallypossessing mechanical or physical propertiessuperior or unique compared to traditional ceramics

3. Glasses - based primarily on silica and distinguishedby their noncrystalline structure

In addition, glass ceramics - glasses transformedinto a largely crystalline structure by heattreatment


Strength Properties of Ceramics

Theoretically, the strength of ceramics should behigher than metals because their covalent and ionicbonding types are stronger than metallic bondingHowever, metallic bonding allows for slip, the basic

mechanism by which metals deform plastically whensubjected to high stressesBonding in ceramics is more rigid and does not

permit slip under stressThe inability to slip makes it much more difficult for

ceramics to absorb stresses


Imperfections inCrystal Structure of Ceramics

Ceramics contain the same imperfections in theircrystal structure as metals - vacancies, displacedatoms, interstitialcies, and microscopic cracks Internal flaws tend to concentrate stresses, especially

tensile, bending, or impactHence, ceramics fail by brittle fracture much more

readily than metalsPerformance is much less predictable due to

random imperfections and processing variations


Compressive Strength of Ceramics

The frailties that limit the tensile strength of ceramicmaterials are not nearly so operative whencompressive stresses are appliedCeramics are substantially stronger in compression

than in tensionFor engineering and structural applications,

designers have learned to use ceramic componentsso that they are loaded in compression rather thantension or bending


Methods to Strengthen Ceramic Materials

Make starting materials more uniformDecrease grain size in polycrystalline ceramic

productsMinimize porosity Introduce compressive surface stressesUse fiber reinforcementHeat treat


Physical Properties of Ceramics

Densityin general, ceramics are lighter than metalsand heavier than polymersMelting temperatures - higher than for most metals

Some ceramics decompose rather than meltElectrical and thermal conductivities - lower than for

metals; but the range of values is greater, so someceramics are insulators while others are conductorsThermal expansion - somewhat less than for metals,

but effects are more damaging because of brittleness


Traditional Ceramics

Based on mineral silicates, silica, and mineral oxidesfound in naturePrimary products are fired clay (pottery, tableware,

brick, and tile), cement, and natural abrasives suchas aluminaProducts and the processes to make them date back

thousands of yearsGlass is also a silicate ceramic material and is

sometimes included among traditional ceramics


Raw Materials for Traditional Ceramics

Mineral silicates, such as clays of variouscompositions, and silica, such as quartz, are amongthe most abundant substances in nature andconstitute the principal raw materials for traditionalceramicsAnother important raw material for traditional

ceramics is aluminaThese solid crystalline compounds have been formed

and mixed in the earths crust over billions of yearsby complex geological processes


Clay as a Ceramic Raw Material

Clays consist of fine particles of hydrous aluminumsilicate

Most common clays are based on the mineralkaolinite, (Al2Si2O5(OH)4)When mixed with water, clay becomes a plastic

substance that is formable and moldableWhen heated to a sufficiently elevated temperature

(firing ), clay fuses into a dense, strong materialThus, clay can be shaped while wet and soft, and

then fired to obtain the final hard product


Silica as a Ceramic Raw Material

Available naturally in various forms, most important isquartz

The main source of quartz is sandstoneLow in cost; also hard and chemically stablePrincipal component in glass, and an important

ingredient in other ceramic products includingwhiteware, refractories, and abrasives


Alumina as a Ceramic Raw Material

Bauxite - most alumina is processed from this mineral,which is an impure mixture of hydrous aluminumoxide and aluminum hydroxide plus similarcompounds of iron or manganese

Bauxite is also the principal source of metallicaluminumCorundum - a more pure but less common form of

Al2O3, which contains alumina in massive amountsAlumina ceramic is used as an abrasive in grinding

wheels and as a refractory brick in furnaces


Traditional Ceramic Products

Pottery and TablewareBrick and tileRefractoriesAbrasives


New Ceramics

Ceramic materials developed synthetically over the lastseveral decadesThe term also refers to improvements in processing

techniques that provide greater control overstructures and properties of ceramic materials In general, new ceramics are based on compounds

other than variations of aluminum silicate, which formmost of the traditional ceramic materialsNew ceramics are usually simpler chemically than

traditional ceramics; for example, oxides, carbides,nitrides, and borides


Oxide Ceramics

Most important oxide new ceramic is aluminaAlthough also included as a traditional ceramic,

alumina is today produced synthetically from bauxite,using an electric furnace methodThrough control of particle size and impurities,

refinements in processing methods, and blendingwith small amounts of other ceramic ingredients,strength and toughness of alumina are improvedsubstantially compared to its natural counterpartAlumina also has good hot hardness, low thermal

conductivity, and good corrosion resistance


Products of Oxide Ceramics

Abrasives (grinding wheel grit)Bioceramics (artificial bones and teeth)Electrical insulators and electronic componentsRefractory brickCutting tool insertsSpark plug barrelsEngineering components


Figure 7.1 - Alumina ceramic components(photo courtesy of Insaco Inc.)


Carbides

Silicon carbide (SiC), tungsten carbide (WC), titaniumcarbide (TiC), tantalum carbide (TaC), and chromiumcarbide (Cr3C2)Although SiC is a man-made ceramic, its production

methods were developed a century ago, and it isgenerally included in traditional ceramics groupWC, TiC, and TaC are valued for their hardness and

wear resistance in cutting tools and other applicationsrequiring these propertiesWC, TiC, and TaC must be combined with a metallic

binder such as cobalt or nickel in order to fabricate auseful solid product


Nitrides

The important nitride ceramics are silicon nitride(Si3N4), boron nitride (BN), and titanium nitride (TiN)Properties: hard, brittle, high melting temperatures,

usually electrically insulating, TiN being an exceptionApplications:

Silicon nitride: components for gas turbines, rocketengines, and melting cruciblesBoron nitride and titanium nitride: cutting tool

material and coatings


Glass

A state of matter as well as a type of ceramicAs a state of matter, the term refers to an amorphous

(noncrystalline) structure of a solid materialThe glassy state occurs in a material when

insufficient time is allowed during cooling from themolten state for the crystalline structure to formAs a type of ceramic, glass is an inorganic,

nonmetallic compound (or mixture of compounds)that cools to a rigid condition without crystallizing


Why So Much SiO2 in Glass?

Because SiO2 is the best glass formerSilica is the main component in glass products,

usually comprising 50% to 75% of total chemistryIt naturally transforms into a glassy state upon

cooling from the liquid, whereas most ceramicscrystallize upon solidification


Other Ingredients in Glass

Sodium oxide (Na2O), calcium oxide (CaO),aluminum oxide (Al2O3), magnesium oxide (MgO),potassium oxide (K2O), lead oxide (PbO), and boronoxide (B2O3)Functions:

Act as flux (promoting fusion) during heatingIncrease fluidity in molten glass for processingImprove chemical resistance against attack by

acids, basic substances, or waterAdd color to the glassAlter index of refraction for optical applications


Glass Products

Window glassContainerscups, jars, bottlesLight bulbsLaboratory glasswareflasks, beakers, glass tubingGlass fibersinsulation, fiber opticsOptical glasses - lenses


Glass-CeramicsA ceramic material produced by conversion of glass into

a polycrystalline structure through heat treatmentProportion of crystalline phase range = 90% to 98%,

remainder being unconverted vitreous materialGrain size - usually between 0.1 - 1.0m (4 and 40

-in), significantly smaller than the grain size ofconventional ceramics

This fine crystal structure makes glass-ceramicsmuch stronger than the glasses from which theyare derivedAlso, due to their crystal structure, glass-ceramics are

opaque (usually grey or white) rather than clear


Processing of Glass CeramicsHeating and forming operations used in glassworking

create product shapeProduct is cooled and then reheated to cause a

dense network of crystal nuclei to form throughoutHigh density of nucleation sites inhibits grain

growth, leading to fine grain sizeNucleation results from small amounts of nucleating

agents in the glass composition, such as TiO2, P2O5,and ZrO2Once nucleation is started, heat treatment is

continued at a higher temperature to cause growth ofcrystalline phases


Advantages of Glass-Ceramics

Efficiency of processing in the glassy stateClose dimensional control over final product shapeGood mechanical and physical properties

High strength (stronger than glass)Absence of porosity; low thermal expansionHigh resistance to thermal shockApplications:

Cooking wareHeat exchangersMissile radomes


Elements Related to Ceramics

CarbonTwo alternative forms of engineering and

commercial importance: graphite and diamondSiliconBoronCarbon, silicon, and boron are not ceramic materials,

but they sometimesCompete for applications with ceramicsHave important applications of their own


Graphite

Form of carbon with a high content of crystalline C inthe form of layersBonding between atoms in the layers is covalent and

therefore strong, but the parallel layers are bonded toeach other by weak van der Waals forcesThis structure makes graphite anisotropic; strength

and other properties vary significantly with directionAs a powder it is a lubricant, but in traditional solid

form it is a refractoryWhen formed into graphite fibers, it is a high

strength structural material


Diamond

Carbon with a cubic crystalline structure with covalentbonding between atoms

This accounts for high hardness Industrial applications: cutting tools and grinding

wheels for machining hard, brittle materials, ormaterials that are very abrasive; also used indressing tools to sharpen grinding wheels that consistof other abrasives Industrial or synthetic diamonds date back to 1950s

and are fabricated by heating graphite to around3000C (5400F) under very high pressures


Figure 7.2 - Synthetically produced diamond powders(photo courtesy GE Superabrasives, General Electric Company)


Silicon

Semi-metallic element in the same periodic table groupas carbonOne of the most abundant elements in Earth's crust,

comprising26% by weightOccurs naturally only as chemical compound - in

rocks, sand, clay, and soil - either as silicon dioxideor as more complex silicate compoundsProperties: hard, brittle, lightweight, chemically

inactive at room temperature, and classified as asemiconductor


Applications and Importance of Silicon

Greatest amounts in manufacturing are in ceramiccompounds (SiO2 in glass and silicates in clays) andalloying elements in steel, aluminum, and copperAlso used as a reducing agent in certain metallurgical

processesOf significant technological importance is pure silicon

as the base material in semiconductor manufacturingin electronicsThe vast majority of integrated circuits produced

today are made from silicon


Boron

Semi-metallic element in same periodic group asaluminumComprises only about 0.001% of Earth's crust by

weight, commonly occurring as minerals borax(Na2B4O7- 10H2O) and kernite (Na2B4O7-4H2O)Properties: lightweight, semiconducting properties,

and very stiff (high modulus of elasticity) in fiber formApplications: B2O3 used in certain glasses, as a

nitride (cBN) for cutting tools, and in nearly pure formas a fiber in polymer matrix composites


Guide to Processing Ceramics

Processing of ceramics can be divided into twobasic categories:1. Molten ceramics - major category of molten

ceramics is glassworking (solidificationprocesses)

2. Particulate ceramics - traditional and newceramics (particulate processing)

Ch07 Ceramics Wiley

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